The present invention relates in general to an organic light-emitting display device; and, more particularly, the invention relates to a display device that is preferable for use in displaying pictures by means of organic light-emitting devices.
Planar type display devices of the type used as man-machine interfaces have received increased attention with the advent of the real multi-media age. Such planar type display devices have used liquid crystal displays; however, liquid crystal display devices have problems in that they have a narrow angle of visibility and low-speed response characteristics.
In recent years, organic light-emitting display devices have been proposed as the next-generation planar type display device. In this regard, organic light-emitting display devices have such characteristics as excellent auto-light-emission, a wide angle of visibility, and a high-speed response.
In such organic light-emitting display devices, pixels are formed by organic light-emitting elements, and the organic light-emitting display device has a structure in which a first electrode, such as ITO, an organic layer comprised of a hole transport layer, a light-emitting layer, an electron transport layer, etc., and a second electrode having a small work function, are provided on a glass substrate.
When a voltage of several volts is applied between the electrodes, holes are injected into the first electrode, whereas electrons are injected into the second electrode, and the holes and electrons pass, respectively, through the hole transport layer or the electron transport layer so as to be coupled with each other in the light-emitting layer, whereby excitons are generated. Light is emitted when the exciton returns to its ground state. The light thus emitted is transmitted through the first electrode, which is being transparent, and is taken out from the back side of the substrate.
The types of display systems using organic light-emitting elements for pixels include simple matrix organic light-emitting display devices and active matrix organic light-emitting display devices.
The simple matrix organic light-emitting display device comprises an organic layer comprised of a hole transport layer, a light-emitting layer, an electron transport layer, etc. provided at positions of intersection of pluralities of anode lines and cathode lines, and each pixel is turned ON for a selected time during one frame period. The selected time is a time width obtained by dividing one frame period by the number of the anode lines. The simple matrix organic light-emitting display device has the advantage of having a simple structure.
However, the selected time is shortened as the number of the pixels increases, so that it is necessary to raise the driving voltage to thereby enhance the instantaneous luminance during the selected time and to bring the average luminance during one frame period to a predetermined value. Thus, there is the problem that the life of the organic light-emitting devices is shortened. In addition, since the organic light-emitting devices are driven by an electric current, the voltage drop due to the wiring resistance is generated, and the voltage cannot be uniformly impressed on each of the pixels, particularly in the case of a large screen, with the result that variations in brightness are produced in the display device. Thus, the simple matrix organic light-emitting display device has limitations as to enhancement of the definition and enlargement of the screen.
On the other hand, in the active matrix organic light-emitting display device, a driving device made up of a switching device composed of two to four thin film transistors and a capacitance is connected to an organic EL (light-emitting) device constituting each pixel, and so full turning-ON during one frame period is possible. Therefore, it is unnecessary to enhance the brightness, and it is possible to prolong the life of the organic light-emitting devices. Accordingly, the active matrix organic light-emitting display device is advantageous from the viewpoint of enhancement of the definition and enlargement of the screen.
In the conventional organic light-emitting display devices, the light being emitted is taken out from the back side of the substrate, and therefore, an aperture ratio is limited in the active matrix organic light-emitting display device in which a driving portion is provided between the substrate and the organic light-emitting device.
In order to solve the above-mentioned problems, attempts are provided to make the upper second electrode transparent and to take out the emitted light from the upper electrode side.
For example, U.S. Pat. No. 5,703,436 discloses an organic EL device in which the upper electrode is constituted of two layers, in which an injection layer of Mg, Ag, etc. is used as a first layer, a transparent electrode of ITO (Indium Tin Oxide), etc. is used as a second layer, and light is taken out from the upper electrode.
In addition, Japanese Patent Laid-open No 6-163158 (1994) discloses an organic EL device comprising an electron injection layer composed of a transparent alkaline earth metal oxide and a transparent cathode material.
Besides, Japanese Patent Laid-open No. 2001-148291 discloses a pixel structure in which a partition wall is formed at an upper portion at the position where an electrode of a driving device and a lower electrode of an organic light-emitting device constituting a pixel are connected in an active matrix organic light-emitting display device. It is also disclosed that this structure is applicable also to a display device in which light is taken out from the upper electrode side.
In the device mentioned above, a transparent conductive film is used as the second electrode to take out light from the upper electrode side. In this case, film formation at a low temperature is indispensable in order not to cause damage to the organic layer functioning as an underlying layer. As a result, the resistance of the film is as high as not less than 300 times in resistivity as compared with a metallic film of Al or the like. In addition, even in the case where the second electrode is constituted of a metallic film, in order to reduce the damage to the organic layer functioning as the underlying layer, it is impossible to enlarge the thickness of the metallic film. Therefore, enlargement of the size of the panel poses a problem with the high resistance of the electrode.
Besides, in the conventional active matrix organic light-emitting display device, current supply lines for connecting the first electrode (anode) and the second electrode (cathode) on the opposite sides of the organic layer of the organic light-emitting device with a power source are formed by use of a metallic film of a driving layer. In this case, the connection between the current supply line connected to a minus terminal of the power source and the second electrode (cathode) of the organic light-emitting device is established through a contact hole formed in an inter-layer insulation film provided in a region free of pixels, for example, in the vicinity of a panel edge.
In other words, the second electrode of the organic light-emitting device belonging to each pixel and the current supply line are connected to each other through the contact hole. In this case, since the contact hole serves as a feeding point and the feeding point and the second electrode of each organic light-emitting device are connected by the current supply line, the resistance of wiring the varies with the distance from the contact hole to the pixel. Therefore, the effective voltage applied to the organic light-emitting device constituting the pixel varies with the wiring resistance, and the luminance value varies according to the position of the pixel.